Radiofrequency (RF) ablation of cardiac and other tissue is a well-known method for creating thermal injury lesions at the tip of an electrode. Radiofrequency current is delivered between a skin (ground) patch and the electrode. Electrical resistance at the electrode-tissue interface results in direct resistive heating of a small area, the size of which depends upon the size of the electrode, electrode tissue contact, and current (density). Further tissue heating results from conduction of heat within the tissue to a larger zone. Tissue heated beyond a threshold of approximately 50-55 degrees C. is irreversibly injured (ablated).
Resistive heating is caused by energy absorption due to electrical resistance. Energy absorption is related to the square of current density and inversely with tissue conductivity. Current density varies with contact area conductivity, voltage and inversely with the square of radius from the ablating electrode. Therefore, energy absorption varies with conductivity, the square of applied voltage, and inversely with the fourth power of radius from the electrode. Resistive heating, therefore, is most heavily influenced by radius, and penetrates a very small distance from the ablating electrode. The rest of the lesion is created by thermal conduction from the area of resistive heating. This imposes a limit on the size of ablation lesions that can be delivered from a surface electrode.
Theoretical methods to increase lesion size would include increasing electrode diameter, increasing the area of electrode contact with tissue, increasing tissue conductivity and penetrating the tissue to achieve greater depth and increase the area of contact, and delivering RF until maximal lesion size has been achieved (60-90 seconds for full maturation).
The electrode can be introduced to the tissue of interest directly (for superficial/skin structures), surgically, endoscopically, laparoscopically or using percutaneous transvascular (catheter-based) access. Catheter ablation is a well-described and commonly performed method by which many cardiac arrhythmias are treated.
Catheter ablation is sometimes limited by insufficient lesion size. Ablation of tissue from an endovascular approach results not only in heating of tissue, but heating of the electrode. When the electrode reaches critical temperatures, denaturation of blood proteins causes coagulum formation. Impedance can then rise and limit current delivery. Within tissue, overheating can cause steam bubble formation (steam “pops”) with risk of uncontrolled tissue destruction or undesirable perforation of bodily structures. In cardiac ablation, clinical success is sometimes hampered by inadequate lesion depth and transverse diameter even when using catheters with active cooling of the tip. Theoretical solutions have included increasing the electrode size (increasing contact surface and increasing convective cooling by blood flow), improving electrode-tissue contact, actively cooling the electrode with fluid infusion, changing the material composition of the electrode to improve current delivery to tissue, and pulsing current delivery to allow intermittent cooling.
To improve electrode-tissue contact, current catheters may have pressure sensors at the distal tip to detect whether the tip electrode is in contact with tissue. However, merely detecting contact does not indicate how much of the tip electrode is actually surrounded by tissue or by fluid and blood. Introduction of an energized electrode into cardiac space results in the formation of a simplified resistive circuit; current flows from the electrode through two parallel resistors via the surrounding blood and the contacting tissue. Understanding the relative surface area of each of these paths will allow for an estimation of each path's respective resistance and therefore the current flow. Such information would be helpful to improve estimation of size and shape of lesions created by ablation, as lesion size and shape are likely a function of power, time and size of contact area of electrode and tissue.
Method and apparatus employing optical spectroscopy for determining tissue attributes are known. For example, U.S. Pat. No. 7,623,906 discloses a method and an apparatus for a diffuse reflectance spectroscopy which includes a specular control device that permits a spectroscopic analyzer to receive diffusely reflected light reflected from tissue. U.S. Pat. No. 7,952,719 discloses an optical catheter configuration combining Raman spectroscopy with optical fiber-based low coherence reflectometry. U.S. Pat. No. 6,377,841 discloses the use of optical spectrometry for brain tumor demarcation.
Accordingly, it is desirable that a catheter be able to assess and measure the amount of contact between an ablation electrode and tissue versus fluid, such as blood, for improving lesion size and depth. It is also desirable that the catheter effectuate such assessment and measurement by optical means that can measure accurately and fit inside the tip electrode without disruption to the function of the tip electrode.